The need for special ceramic materials is steadily increasing all over the world. Among these materials, alumina exhibits some of the most superior properties of all ceramic materials: for example, high melting point and excellent wear resistance, insulation, mechanical strength and chemical stability. Thus, alumina has become one of the most popular ceramic materials in a diverse array of applications. Furthermore, alumina is easily obtained and the processing technique for making it is standardized, so it has been employed from the 1900's in the large-scale production of high-temperature refractories, insulating materials, grinding media, cutting tools, spark plugs, integrated circuit (IC) substrates, artificial tooth implants, high-voltage sodium lamps with light perviousness, catalytic materials, composite materials in dispersion phase and so on. Consequently, alumina has become one of the materials required in the processes of light and heavy industries.
The production of alumina powder having high specific surface area, which is commonly applied as a desiccants and an adsorbent for gases and organic fluids, began from the 1930's, when it was also called activated alumina. Moreover, the alumina powder having high specific surface area is applied in separating components of chemical engineering processes and treating water. Owing to alumina having chemically and thermally stable properties, the gibbsite or boehmite obtained in the Bayer process undergoes a thermal treatment for forming α-phase alumina, and various alumina transition phases, such as κ-phase, θ-phase, δ-phase and γ-phase, which are derived from the formation process of the α-phase alumina, have also become the most popular catalytic materials or catalyst carriers applied in the chemical industry. At present, alumina material serving as a catalyst can be a film coated on the surface of the carrier (such as in a catalytic converter of a car), or it can also be a sphere, a cylinder, a flake or other shape, depending on the actual requirement.
It is a need to develop alumina powders with high specific surface area that can be resistant to higher temperature environment (above 900° C.), for example, that can be used for automotive emission control systems. Monolithic catalyst systems consisting of a cellular ceramic coated with high surface area (γ-) alumina and noble metal catalysts are now widely used. The catalyst system was developed in the early 1970 and commercially utilized starting with 1975 model-year cars in US based on the Clean Air Act of 1970. For this reason, the industry at that time was required to more exactly control the engine design and the fuel/air ratio during exploring the power fuel. Moreover, a system for treating exhaust gases was further disposed on the exhaust pipe connected with the engine, for treating the exhaust gases before exhausting, so as to reduce the amount of harmful gases. The above system for treating exhaust gases is now called the “catalytic converter”. In the typical process, the honeycomb-like porous monolithic carrier, which is the cordierite-based material, is firstly produced, and then the pore walls of the carrier are coated with a film that contains noble metal micro-particles of palladium (Pd), platinum (Pt) and rhodium (Rh) in γ-phase, δ-phase and θ-phase alumina, for example. The transition alumina phases are obtained by thermally treating boehmite, and they serve as the carriers for the catalytic material of Pd, Pt and Rh metal micro-particles. In addition, because of increasing environmental protection requirements and more stringent regulations for reducing harmful gases, the functional requirements for catalytic converters of exhaust gases is also increasing.
With 2005 as the deadline, there is an essential need for obtaining a material for converting automobile exhaust that can be maintained at the desired high specific surface area when suffering higher temperatures such as 900 to 1000 degrees Celsius (° C.). The commercial high-temperature catalytic alumina (Al2O3) materials for automobile emission control include various transition alumina phases that are mainly derived from boehmite. These transition alumina phases will undergo the route from boehmite to γ-, δ-, θ-, then finally to α-alumina, in which the final α-Al2O3 may be formed at the temperature around 1000˜1100° C. However, when the α-Al2O3 formation occurs, it usually accompanies with a drastic surface area reduction of the catalytic alumina material, resulting in the function deterioration of the catalyst converter. The working temperature of catalyst converter is normally above 800° C., and more recently reaching 1000° C. to 1100° C. for meeting the new State levels. In this case, the catalytic alumina material has serious surface area reduction problem at such temperature range and cannot meet the market requirement. Thus various methods have been employed to retard the surface area reduction, extending its lifetime to meet the environmental regulations.
U.S. Patent Application No. 20040043898 discloses a catalyst carrying a catalyst material containing an alkaline metal and/or an alkaline earth metal on a carrier and used as an NOx trap catalyst for purifying automobile exhaust gas and the like comprises alumina incorporated into the carrier and/or placed between the carrier and the catalyst material, thereby suppressing the deterioration of the carrier caused by the metals such as Li, Na, K and Ca to be used as an alkaline metal and/or an alkaline earth metal and enabling it to be used for a extended period of time.
U.S. Pat. No. 6,846,466 discloses a catalyst for purifying an exhaust gas, which includes an upstream side catalyst and a downstream side catalyst. The upstream side catalyst is disposed on an upstream side with respect to an exhaust gas flow, and the downstream side catalyst is disposed on a downstream side with respect thereto. The upstream side catalyst includes a first loading layer, being composed of an alumina containing Ba and La at least, and a first noble metal, being held by the first loading layer and being at least one member selected from the group consisting of Pd, Pd and Rh and Pd and Pt. Alternatively, in addition to the aluminum, the first loading layer can be composed of Ce, a solid solution of Ce and Zr and a solid solution of Ce, Zr and Y in an amount as less as possible. The downstream side catalyst includes a second loading layer, being composed of at least one member selected from the group consisting of an alumina containing La, Ce, a solid solution of Ce and Zr and a solid solution of Ce, Zr and Y, and a second noble metal, being held by the second loading layer and being composed of at least one member selected from the group consisting of Pt, Pd and Rh.
U.S. Pat. No. 6,623,716 discloses an exhaust gas purifying catalyst for purifying exhaust gas discharged from an automotive internal combustion engine. The exhaust gas purifying catalyst comprises at least one noble metal selected from the group consisting of platinum, palladium and rhodium; and boehmite alumina serving as a base material. In this exhaust gas purifying catalyst, nitrogen oxides in exhaust gas from the engine is trapped to the exhaust gas purifying catalyst when exhaust gas is in a lean region and is reduced into nitrogen by the exhaust gas purifying catalyst when exhaust gas is in a stoichiometric region or a rich region.
U.S. Pat. No. 5,439,865 discloses a catalyst for exhaust gas purification, which is hereby incorporated by reference. The catalyst for exhaust gas purification comprises a heat-resistant inorganic monolith carrier and a catalyst layer loaded thereon. The catalyst layer includes a catalyst composition containing at least one noble metal selected from Pt, Pd and Rh, as an active catalyst component, and active alumina. The catalyst composition has a specific surface area of at least 50 m2/g and a porosity of at least 50%. This catalyst for exhaust gas purification contains noble metal(s) in a well dispersed state, has excellent high-temperature durability, and is low in thermal deterioration of catalyst performance. Hence, the catalyst can be suitably used as a converter installed in engine manifolds of gasoline engine automobiles, or as a heater having improved purification ability for the exhaust gases emitted from automobiles during their cold start.
U.S. Pat. No. 4,780,447 discloses a catalyst, which is capable of controlling not only HC, CO and NOx, but also H2S emission from the tail pipe of catalytic converter-equipped automobiles, which is hereby incorporated by reference. The catalyst is made of noble metals promoted with ceria-rich rare earth oxides, preferably doubly promoted along with alkali metal oxides, and oxides of nickel and/or iron as an H2S gettering ingredient. The oxides of nickel and/or iron are present in an H2S gettering effective amount and in an amount up to 10 wt %. The alumina support can have additionally from 0 to 20% SiO2 present.
However, the commercial catalyst substrates cannot be used under such high temperatures for a long time. The reason is described as above, which is mainly that crystallite size growth accompanied with phase transformations of the transition alumina occurs at such high temperatures, resulting in the rapid reduction in the specific surface area of the alumina substrate. As the catalysis area for the exhaust gases per unit time is decreased, the catalyst substrate suffers a substantial loss of its catalyzing function, followed by the shortened lifetime of the converter.
Accordingly, as for the catalytic converter of the car, there is a need for an alumina carrier material capable of maintaining its high specific surface area when suffering high temperatures for a long time, so as to satisfy the further requirement of the new generation.